Recently, magneto-optical memory devices such as magneto-optical disks have received much attention as memory devices having great densities and large capacities that are capable of rewriting information. Among the magneto-optical memory devices, those which are capable of light-modulation overwriting have been demanded year after year because they make it possible to rewrite information without erasing former information, to enable two-sided recording, and to be easily adapted for use in multi-beam operation.
As described in Jpn. J. Appl. Phys., Vol. 26(1987) Suppl. p. 155-159, the magneto-optical memory device that is capable of light-modulation overwriting has a recording layer and an auxiliary layer that are constituted of perpendicularly magnetized films.
FIG. 13 shows a magneto-optical disk apparatus, which is one example of magneto-optical recording apparatus for use with the magneto-optical memory device of this type.
The magneto-optical disk apparatus is provided with an objective lens 27 for converging a light beam 26 onto a magneto-optical disk, a magnet 24 for use in initialization, and a magnet 25 used for recording.
The magneto-optical disk has an arrangement wherein a recording layer 22 and an auxiliary layer 23 are laminated on a substrate 21. The recording layer 22 has a higher coercive force and a lower Curie temperature than the auxiliary layer 23, and is used for maintaining information and reading information. The auxiliary layer 23 is used for copying information onto the recording layer 22 and erasing information from the recording layer 22 by utilizing an exchange coupling force that is exerted between the recording layer 22 and the auxiliary layer 23.
Upon conducting a light-modulation overwriting operation, the auxiliary layer 23 is first initialized. That is, the magnetization direction of the auxiliary layer 23 is uniformly arranged to one direction by applying an initializing magnetic field from the magnet 24. Then, information is rewritten through the overwriting operation by modulating the intensity of the light beam 26 into high and low levels while applying a writing magnetic field having the direction opposite to the initializing magnetic field from the magnet 25 to an area which is being irradiated by the light beam 26.
More specifically, when the light beam 26 of the high level is projected, the temperature of a portion of the auxiliary layer 23 located at the area irradiated by the light beam 26 rises up to the vicinity of the Curie temperature (T.sub.c3), thereby allowing the magnetization direction to be reversed by the writing magnetic field. When the irradiated area of the light beam 26 is shifted due to the rotation of the magneto-optical disk, the above-mentioned portion of the auxiliary layer 23 cools off. When the temperature of the portion of the auxiliary layer 23 drops to the vicinity of the Curie temperature (T.sub.c1) of the recording layer 22, the magnetization direction of the recording layer 22 becomes coincident with that of the auxiliary layer 23 due to an exchange coupling force that is exerted on an interface between the recording layer 22 and the auxiliary layer 23.
In the case of projecting the light beam 26 of the low level, the temperature of the auxiliary layer 23 rises up to merely the vicinity of T.sub.c1. Therefore, the magnetization direction of the auxiliary layer 23 is not reversed by the writing magnetic field. The magnetization direction of the recording layer 22 becomes coincident with that of the auxiliary layer 23 in the same manner as described above.
As described above, the conventional magneto-optical disk apparatus has achieved the realization of light-modulation overwriting operation that is carried out on the recording layer 22 by the use of the auxiliary layer 23.
In contrast, upon reproducing information thus recorded, the rotation of polarization plane, which is caused by the magneto-optical effect, is detected by applying the light beam 26 having a smaller intensity than that used in recording.
However, in the conventional light-modulation overwriting method, there arises a problem wherein two magnets 24 and 25 have to be installed since the initializing magnetic field and the writing magnetic field are required.
Moreover, since the intensity of the magnetic field required for the initialization ranges from 400 to 500 kA/m, it is necessary to install a large magnet 24 for the initialization. This causes an adverse effect on making the magneto-optical disk apparatus more compact and thinner.